1. Field of the Invention
The present invention relates generally to methods of decomposing layout design for preparing a photomask set printed onto a wafer by photolithography, forming a photomask set, and fabricating an integrated circuit, and more specifically to the above methods that decompose a layout design to form a photomask set, and then align the overlapping photomasks.
2. Description of the Prior Art
An integrated circuit (IC) is a device, such as a semiconductor device, or an electronic system that includes many electronic components, such as transistors, resistors and diodes. These components are often interconnected to form multiple circuit components, e.g. gates, cells, memory units, arithmetic units, controllers and decoders. An IC includes multiple layers of wiring that interconnects the electronic and circuit components. Design engineers design ICs by transforming logical or circuit descriptions of the components into geometric descriptions, which are called design layouts.
Fabrication foundries (fabs) manufacture ICs based on the design layouts using a photolithographic process. Photolithography is an optical printing and fabrication process by which patterns on a photolithographic mask (i.e. a photomask) are imaged and defined onto a photosensitive layer coating a substrate. To fabricate an IC, photomasks are created using the IC design layout as a template. The photomasks contain the various geometries (features) of the IC design layout. The various geometries contained on the photomasks correspond to the various base physical IC elements that comprise functional circuit components such as transistors, interconnect wiring and via pads, as well as other elements that are not functional circuit elements, but are used to facilitate, enhance or track various manufacturing processes. Through sequential use of the various photomasks corresponding to a given IC in an IC fabrication process, a large number of material layers of various shapes and thicknesses with different conductive and insulating properties may be built up to form the overall IC and the circuits within the IC design layout.
Constraining factors in traditional photolithographic processes limit their effectiveness as circuit complexity continues to increase and transistor designs become more advanced and ever smaller in size. Some constraining factors are the lights/optics used within the photolithographic processing systems. Specifically, the lights/optics are band limited due to physical limitations (e.g. wavelength and aperture) of the photolithographic process. The photolithographic process therefore cannot print beyond a certain pitch and distance, and also suffers from other physical manufacturing constraints.
A pitch specifies a sum of the width of a feature and the space on a side of the feature separating the feature from a neighboring feature. Depending on the photolithographic process, factors such as optics and wavelengths of light or radiation restrict how small the pitch may be made before features can no longer be reliably printed to a wafer or mask. The smallest size of any feature that can be created on a wafer is severely limited by the pitch.
With the advance of ultra-deep submicron technology, the feature size and feature pitch become so small that existing lithography processes meet a bottleneck in printing the shapes represented by the features. There are also difficulties in the practical use of advanced photolithographic processes, such as extreme ultra violet (EUV). Current lithography technology is expected to be used for next generation silicon technology. To compensate for the difficulty in printing the shape of small pitches, multiple patterning lithography is recognized as a promising solution for 32 nm, 22 nm and sub-22 nm volume IC production. Multiple patterning lithography technology decomposes a single layer of a layout into multiple masks and applies multiple exposures to print the shapes in the layer. The decomposition provided by multiple patterning lithography increases shape printing pitch and improves the focus depth.